Wet-Chemical Etching
of Silicon
Revised: 2013-11-07
Source:
www.microchemicals.com/downloads/application_notes.html
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MicroChemicals GmbH - Silicon Etching
Anisotropic Silicon Etching
Strong alkaline substances (pH > 12) such as aqueous KOH- or TMAH-solutions etch Si via
Si + 4 OH- Æ Si(OH)4 + 4e-.
Since the bonding energy of Si atoms is different for each crystal plane, and KOH/TMAH Si
etching is not diffusion- but etch rate limited, Si etching is highly anisotropic: While the {100}and {110}-crystal planes are being etched, the stable {111} planes act as an etch stop:
(111)-orientated Si-wafers are almost not attacked by the etch.
(100)-orientated wafers form square-based pyramids with {111} surfaces. These pyramids
are realised on c-Si solar cells for the purpose of reflection minimization.
(110)-orientated wafers form perpendicular trenches with {111} side-walls, used as e. g.
microchannels in micromechanics and microfluidics.
The degree of anisotropy (= etch rate selectivity between different crystal planes), the etch
rates, and the etching homogeneity depend on the etching temperature, atomic defects in the
silicon crystal, intrinsic impurities of the Si crystal, impurities (metal ions) by the etchant, and
the concentration of Si[100]
atoms already etched.
The doping concentration of the Si to be
[111]
etched also strongly impacts on the etching:
During etching, Boron
{111}
doped
Si
forms
borosilicate glass on the
surface which acts as
etch stop if the boron
doping concentration
exceeds 1019 cm-3.
The following table lists
etch rates of Si and the
hard masks SixNy and
[100]
[010]
SiO2, and etch selectivity between different
[110]
crystal planes as a function of the etchant.
Anisotropic
etching
{110}
{100}
[100]
[110]
{111}
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MicroChemicals GmbH - Silicon Etching
Isotropic Etching of Silicon and SiO2
Etchant
KOH
(44%, 85°C)
TMAH
(25%, 80°C)
EDP
(115°C)
Etch rate ratio
(100)/(111) (110)/(111)
300
600
37
68
20
10
Etch rate (absolute)
(100)
Si3N4
SiO2
Advantages (+)
Disadvantages (-)
(-) Metal ion containing
(+) Strongly anisotropic
(-) Weak anisotropy
0.3-1
<1 Å/min 2 Å/min
mm/min
(+) Metal ion free
(-) Weak anisotropy , toxic
1.25 mm/min 1 Å/min 2 Å/min (+) Metal ion free, metallic
hard masks possible
1.4 mm/min <1 Å/min 14 Å/min
The following chemical reactions summarize the basic etch mechanism for isotropic silicon
etching (steps 1-4), and SiO2 (only step 4) using a HF/HNO3 etching mixture:
(1) NO2 formation (HNO2 always in traces in HNO3):
HNO2 + HNO3
Æ 2 NO2 + H2O
(2) Oxidation of silicon by NO2:
2 NO2 + Si
Æ Si2+ + 2 NO2(3) Formation of SiO2:
Si2+ + 2 (OH)- Æ SiO2 + H2
(4) Etching of SiO2:
SiO2 + 6 HF
Æ H2SiF6 + 2H2O
In conclusion, HNO3 oxidises Si, and HF etches the SiO2 hereby formed.
Fig. right-hand: High HF : HNO3 ratios promote rate-limited etching (strong temperature dependency of the etch rate) of Si via the oxidation (1) - (3),
while low HF : HNO3 ratios promote diffusion-limited etch[HF]
Increasing etch
ing (lower temperature dependency of the etch rate) via
rate temperastep (4). HNO3-free HF etches do not attack Si.
ture dependThe SiO2 etch rate is determined by the HF-concentraency
tion, since the oxidation (1) - (3) does not account.
Compared to thermal oxide, deposited (e. g. CVD)
IncreasSiO2 has a higher etch rate due to its porosity; wet
ing selecoxide a slightly higher etch rate than dry oxide for
tivity Si/
the same reason.
SiO2
An accurate control of the etch rate requires a
es
s
a
temperature control within ± 0.5°C. Dilution with
re
inc
acidic acid improves wetting of the hydrophobic
e
t
ra
Si-surface and thus increases and homogenizes
h
c
et
the etch rate.
Si
Doped (n- and p-type) silicon as well as phos[H2O]+[CH3COOH]
[HNO3]
phorus-doped SiO2 etches faster than undoped Si or SiO2.
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best of our knowledge. However, we cannot issue any guarantee concerning the accuracy of
the information.
We assume no liability for any hazard for staff and equipment which might stem from the
information given in this brochure.
Generally speaking, it is in the responsibility of every staff member to inform herself/himself
about the processes to be performed in the appropriate (technical) literature, in order to
minimize any risk to man or machine.
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